In general, integrated circuit devices are usually sealed in a ceramic package (including a method known as cerdip, in which the ceramic package is sealed with glass), a plastic package, or a similar type of package. The package normally comprises a base carrying an integrated circuit device and a cover. Particularly when a ceramic package is used, the material of the package contains naturally occuring trace amounts of uranium and thorium in concentrations of the order of several ppm, as reported by T. C. May and M. J. Woods in "A New Physical Mechanism for Soft Errors in Dynamic Memories", IEEE Reliability Physics Symp., San Diego, Calif., April 1978. It is known that these impurities emit alpha-particles and, when present in the cover, are a major cause of soft errors in dynamic RAM (Random Access Memory) and CCD (Charged Coupled Device) memories. The problems caused by alpha-particle-emission are due to the materials lying above the active region of the integrated circuit device, in the cover. The alpha-particles generated by the ceramic material below the integrated circuit device must pass completely through the interior of the integrated circuit device in order to get to the active region and, therefore, are not troublesome since the excursion range of alpha-particles in the device is usually less than the thickness of the device.
Powders such as silica are used as fillers in plastic packaging materials with the same impurities being present. In these plastic packages the level of impurities in the filler is such that the alpha-particles emitted from the plastic packaging material above the encapsulated integrated circuit device, is still unacceptably high in spite of the fact that some of the alpha-particles emitted from the filler material are absorbed by the plastic. In addition, such plastic materials have the disadvantages of comparatively low moisture and heat resistance. It is therefore difficult to employ plastic encapsulated integrated circuit devices in high reliability systems. Hence, high reliability systems utilize ceramic packaging.
The prior art has attempted to reduce soft errors in ceramic packaging by purifying the packaging material to a high degree so that it does not contain either uranium or thorium (IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-2, No. 4, December 1979). Usually, however, the packaging material is made of a composite material consisting of several raw materials and may contain impurities introduced in the various manufacturing steps. Thus, it has been extremely difficult to fabricate a package which does not contain uranium or thorium impurities.
Alternatively, a variety of curable liquid coatings including silicones, epoxies and organic polyimide have been applied directly to the integrated circuit devices to protect them from alpha-particle-generated soft errors (UK Patent Application No. 2,036,428). This approach presents several problems. Differing thermal expansion coefficients of the coating and the integrated circuit device can result in the coating pulling away from the integrated circuit device during the curing process or later in the field. Additionally, the differing thermal expansion coefficients may result in the bonding wires coming loose during curing or later use. Other concerns include the potential for contamination or corrosion of the integrated circuit device and difficulty in achieving a uniform coating thickness.
Machine-cut polyimide films have been used to cover the integrated circuit device by adhering the film to the device with an adhesive (Electronics, Sept. 11, 1980, p. 41). These films can be reproducibly made to the required thickness and since they are form-fitted can avoid the problems encountered with liquid application techniques in which the coating material covers the bonding area on the integrated circuit device. However, potential problems such as misalignment and adhesion breakdown in the field make this technique less than ideal.